This invention relates to a security system for the protection of computer programs, and more particularly to such a system which employs a software protection hardware device which is plugged into a communications port of a computed and distinguishes a disguised predetermined code hidden within a stream of signals emanating from the computer by comparing the disguised predetermined code with a stored reference code which itself may be changed and used to decode the computer transmission under the control of the software to be protected.
Hardware based software protection devices have been proposed in which the security device is plugged into a communications port of a computer via an interface connector through which signals to and from the computer are exchanged. One example is U.S. Pat. No. 4,562,306 to the inventors of the present application, in which an actively coded hardware device is used to protect computer software. In this patent the hardware security device is attached to an interface connector which is plugged in to a communications port of a computer through which signals pass from the computer to the security device under the control of the software, and other signals from the security device to the computer as a response to the signals applied from the computer.
In such devices the hardware security device is usually encapsulated in plastic or epoxy in order to further enhance the security making it difficult or impossible to discover the code without tearing the device apart. However, the security may be compromised by observing the signals to and from the security device regardless of their nature by simply monitoring and studying such signals using reverse engineering to eventually break down the code or functioning of the hardware device. Accordingly, the security of the plugged in device for the software program which it is desired to protect will be primarily a function of how difficult it is to fund out what the hardware device is doing and what signals it is operating on to provide the protection. The present invention is directed to providing a deterrent which can be configured in such a manner that any attempt to decipher or emulate the signals may be deemed impractical.